Thecodontosaurus (meaning "socket-toothed lizard") was a very early dinosaur that lived during the Upper Triassic, a time when the Earth was relatively warm and much of the land was dry and desert-like. This was around the time that the dinosaurs were just starting to appear.

Fossils of Thecodontosaurus have been found in southwest England (near Bristol) and Wales, which were probably dry and desert-like when Thecodontosaurus lived. Thecodontosaurus was first described by H. Riley and S. Stutchbury in 1836, and named by Morris in 1843.

Modern cladistic work has not been conclusive. Thecodontosaurus was earlier put under the Prosauropoda.[1] More recently was suggested that Thecodontosaurus and its relatives came before the prosauropod-sauropod split.[2]Thecodontosaurus is sometimes placed in a very basal position among the sauropodomorph dinosaurs. New reconstructions show that its neck is proportionally shorter than those in more 'advanced' early sauropodomorphs.